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    <h1 align="CENTER">Module mod_isapi</h1>

    <p>This module supports ISAPI Extensions within Apache for
    Windows.</p>

    <p><a href="module-dict.html#Status"
    rel="Help"><strong>Status:</strong></a> Base<br />
     <a href="module-dict.html#SourceFile"
    rel="Help"><strong>Source File:</strong></a> mod_isapi.c<br />
     <a href="module-dict.html#ModuleIdentifier"
    rel="Help"><strong>Module Identifier:</strong></a>
    isapi_module<br />
     <a href="module-dict.html#Compatibility"
    rel="Help"><strong>Compatibility:</strong></a> WIN32 only</p>

    <h2>Summary</h2>

    <p>This module implements the Internet Server extension API. It
    allows Internet Server extensions (<em>e.g.</em> ISAPI .dll
    modules) to be served by Apache for Windows, subject to the
    noted restrictions.</p>

    <p>ISAPI extension modules (.dll files) are written by third
    parties. The Apache Group does not author these modules, so we
    provide no support for them. Please contact the ISAPI's author
    directly if you are experiencing problems running their ISAPI
    extention. <strong>Please <em>do not</em> post such problems to
    Apache's lists or bug reporting pages.</strong></p>

    <h2>Directives</h2>

    <ul>
      <li><a
      href="#isapireadaheadbuffer">ISAPIReadAheadBuffer</a></li>

      <li><a
      href="#isapilognotsupported">ISAPILogNotSupported</a></li>

      <li><a
      href="#isapiappendlogtoerrors">ISAPIAppendLogToErrors</a></li>

      <li><a
      href="#isapiappendlogtoquery">ISAPIAppendLogToQuery</a></li>
    </ul>

    <h2>Usage</h2>

    <p>In the server configuration file, use the AddHandler
    directive to associate ISAPI files with the
    <code>isapi-isa</code> handler, and map it to the with their
    file extensions. To enable any .dll file to be processed as an
    ISAPI extention, edit the httpd.conf file and add the following
    line:</p>
<pre>
    AddHandler isapi-isa .dll
</pre>

    <p>ISAPI extensions are governed by the same permissions and
    restrictions as CGI scripts. That is, <code>Options
    ExecCGI</code> must be set for the directory that contains the
    ISAPI .dll file.</p>

    <p>Review the <a href="#notes">Additional Notes</a> and the <a
    href="#journal">Programmer's Journal</a> for additional details
    and clarification of the specific ISAPI support offered by
    mod_isapi.</p>

    <h2><a id="notes" name="notes">Additional Notes</a></h2>

    <p>Apache's ISAPI implementation conforms to all of the ISAPI
    2.0 specification, except for the "Microsoft-specific"
    extensions dealing with asynchronous I/O. Apache's I/O model
    does not allow asynchronous reading and writing in a manner
    that the ISAPI could access. If an ISA tries to access
    unsupported features, including async I/O, a message is placed
    in the error log to help with debugging. Since these messages
    can become a flood, a new directive; <code>ISAPILogNotSupported
    Off</code>, is introduced in Apache 1.3.13.</p>

    <p>Some servers, like Microsoft IIS, load the ISA into the
    server, and keep it loaded until memory usage is too high, or
    specific configuration options are used. Apache currently loads
    and unloads the ISA for each request. This is inefficient, but
    Apache's request model makes this method the only method that
    currently works. Apache 2.0 is expected to support more
    effective loading and caching methods, with more precise
    control over individual ISAPI modules and directories.</p>

    <p>Also, remember that while Apache supports ISAPI Extensions,
    it <strong>does not support ISAPI Filters.</strong> Support for
    filters may be added at a later date, but no support is planned
    at this time.</p>

    <h2><a id="journal" name="journal">Programmer's
    Journal</a></h2>

    <p>If you are programming Apache 1.3 mod_isapi modules, you
    must limit your calls to ServerSupportFunction to the following
    directives:</p>

    <dl>
      <dt>HSE_REQ_SEND_URL_REDIRECT_RESP</dt>

      <dd>Redirect the user to another location.<br />
       This must be a fully qualified URL (e.g.
      http://server/location).</dd>

      <dt>HSE_REQ_SEND_URL</dt>

      <dd>Redirect the user to another location.<br />
       This cannot be a fully qualified URL, you are not allowed to
      pass the protocol or a server name (e.g. simply
      /location).<br />
       This redirection is handled by the server, not the
      browser.<br />
       <strong>Warning:</strong> in their recent documentation,
      Microsoft appears to have abandoned the distinction between
      the two HSE_REQ_SEND_URL functions. Apache continues to treat
      them as two distinct functions with different requirements
      and behaviors.</dd>

      <dt>HSE_REQ_SEND_RESPONSE_HEADER</dt>

      <dd>Apache accepts a response body following the header if it
      follows the blank line (two consecutive newlines) in the
      headers string argument. This body cannot contain NULLs,
      since the headers argument is NULL terminated.</dd>

      <dt>HSE_REQ_DONE_WITH_SESSION</dt>

      <dd>Apache considers this a no-op, since the session will be
      finished when the ISAPI returns from processing.</dd>

      <dt>HSE_REQ_MAP_URL_TO_PATH</dt>

      <dd>Apache will translate a virtual name to a physical
      name.</dd>

      <dt>HSE_APPEND_LOG_PARAMETER <em>Apache 1.3.13 and
      later</em></dt>

      <dd>
        This logged message may be captured in any of the following
        logs: 

        <ul>
          <li>in the \"%{isapi-parameter}n\" component in a
          CustomLog directive</li>

          <li>in the %q log component with the
          ISAPIAppendLogToQuery On directive</li>

          <li>in the error log with the ISAPIAppendLogToErrors On
          directive</li>
        </ul>
        The first option, the %{isapi-parameter}n component, is
        always available and prefered.
      </dd>

      <dt>HSE_REQ_IS_KEEP_CONN <em>Apache 1.3.13 and
      later</em></dt>

      <dd>Will return the negotiated Keep-Alive status.</dd>

      <dt>HSE_REQ_SEND_RESPONSE_HEADER_EX <em>Apache 1.3.13 and
      later</em></dt>

      <dd>Will behave as documented, although the fKeepConn flag is
      ignored.</dd>

      <dt>HSE_REQ_IS_CONNECTED <em>Apache 1.3.13 and
      later</em></dt>

      <dd>Will report false if the request has been aborted.</dd>
    </dl>

    <p>Apache returns FALSE to any unsupported call to
    ServerSupportFunction, and sets the GetLastError value to
    ERROR_INVALID_PARAMETER.</p>

    <p>Prior to Apache 1.3.13, ReadClient was a noop, and any
    request with a request body greater than 48kb was rejected by
    mod_isapi. As of Apache 1.3.13, ReadClient now retrieves the
    request body exceeding the initial buffer (defined by
    ISAPIReadAheadBuffer). Based on the ISAPIReadAheadBuffer
    setting (number of bytes to buffer prior to calling the ISAPI
    handler) shorter requests are sent complete to the extension
    when it is invoked. If the request is longer, the ISAPI
    extension must use ReadClient to retrieve the remaining request
    body.</p>

    <p>WriteClient is supported, but only with the HSE_IO_SYNC flag
    or no option flag (value of 0). Any other WriteClient request
    will be rejected with a return value of FALSE, and a
    GetLastError value of ERROR_INVALID_PARAMETER.</p>

    <p>GetServerVariable is supported, although extended server
    variables do not exist (as defined by other servers.) All the
    usual Apache CGI environment variables are available from
    GetServerVariable. As of Apache 1.3.13, the ALL_HTTP and
    ALL_RAW and variables are now available.</p>

    <p>Apache 2.0 mod_isapi may support additional features
    introduced in later versions of the ISAPI specification, as
    well as limited emulation of async I/O and the TransmitFile
    semantics. Apache 2.0 may also support caching of ISAPI .dlls
    for performance. No further enhancements to the Apache 1.3
    mod_isapi features are anticipated.</p>
    <hr />

    <h2><a id="isapireadaheadbuffer"
    name="isapireadaheadbuffer">ISAPIReadAheadBuffer
    directive</a></h2>

    <a href="directive-dict.html#Syntax"
    rel="Help"><strong>Syntax:</strong></a> ISAPIReadAheadBuffer
    <em>size</em><br />
     <a href="directive-dict.html#Default"
    rel="Help"><strong>Default:</strong></a> 49152<br />
     <a href="directive-dict.html#Context"
    rel="Help"><strong>Context:</strong></a> server config<br />
     <a href="directive-dict.html#Override"
    rel="Help"><strong>Override:</strong></a> None<br />
     <a href="directive-dict.html#Status"
    rel="Help"><strong>Status:</strong></a> Base<br />
     <a href="directive-dict.html#Module"
    rel="Help"><strong>Module:</strong></a> mod_isapi<br />
     <a href="module-dict.html#Compatibility"
    rel="Help"><strong>Compatibility:</strong></a> Apache 1.3.13
    and later, Win32 only 

    <p>Defines the maximum size of the Read Ahead Buffer sent to
    ISAPI extentions when they are initally invoked. All remaining
    data must be retrieved using the ReadClient callback; some
    ISAPI extensions may not support the ReadClient function. Refer
    questions to the ISAPI extention's author.</p>
    <hr />

    <h2><a id="isapilognotsupported"
    name="isapilognotsupported">ISAPILogNotSupported
    directive</a></h2>

    <a href="directive-dict.html#Syntax"
    rel="Help"><strong>Syntax:</strong></a> ISAPILogNotSupported
    on|off<br />
     <a href="directive-dict.html#Default"
    rel="Help"><strong>Default:</strong></a> on<br />
     <a href="directive-dict.html#Context"
    rel="Help"><strong>Context:</strong></a> server config<br />
     <a href="directive-dict.html#Override"
    rel="Help"><strong>Override:</strong></a> None<br />
     <a href="directive-dict.html#Status"
    rel="Help"><strong>Status:</strong></a> Base<br />
     <a href="directive-dict.html#Module"
    rel="Help"><strong>Module:</strong></a> mod_isapi<br />
     <a href="module-dict.html#Compatibility"
    rel="Help"><strong>Compatibility:</strong></a> Apache 1.3.13
    and later, Win32 only 

    <p>Logs all requests for unsupported features from ISAPI
    extentions in the server error log. While this should be turned
    off once all desired ISAPI modules are functioning, it defaults
    to on to help administrators track down problems.</p>
    <hr />

    <h2><a id="isapiappendlogtoerrors"
    name="isapiappendlogtoerrors">ISAPIAppendLogToErrors
    directive</a></h2>

    <a href="directive-dict.html#Syntax"
    rel="Help"><strong>Syntax:</strong></a> ISAPIAppendLogToErrors
    on|off<br />
     <a href="directive-dict.html#Default"
    rel="Help"><strong>Default:</strong></a> off<br />
     <a href="directive-dict.html#Context"
    rel="Help"><strong>Context:</strong></a> server config<br />
     <a href="directive-dict.html#Override"
    rel="Help"><strong>Override:</strong></a> None<br />
     <a href="directive-dict.html#Status"
    rel="Help"><strong>Status:</strong></a> Base<br />
     <a href="directive-dict.html#Module"
    rel="Help"><strong>Module:</strong></a> mod_isapi<br />
     <a href="module-dict.html#Compatibility"
    rel="Help"><strong>Compatibility:</strong></a> Apache 1.3.13
    and later, Win32 only 

    <p>Record HSE_APPEND_LOG_PARAMETER requests from ISAPI
    extentions to the server error log.</p>
    <hr />

    <h2><a id="isapiappendlogtoquery"
    name="isapiappendlogtoquery">ISAPIAppendLogToQuery
    directive</a></h2>

    <a href="directive-dict.html#Syntax"
    rel="Help"><strong>Syntax:</strong></a> ISAPIAppendLogToQuery
    on|off<br />
     <a href="directive-dict.html#Default"
    rel="Help"><strong>Default:</strong></a> off<br />
     <a href="directive-dict.html#Context"
    rel="Help"><strong>Context:</strong></a> server config<br />
     <a href="directive-dict.html#Override"
    rel="Help"><strong>Override:</strong></a> None<br />
     <a href="directive-dict.html#Status"
    rel="Help"><strong>Status:</strong></a> Base<br />
     <a href="directive-dict.html#Module"
    rel="Help"><strong>Module:</strong></a> mod_isapi<br />
     <a href="module-dict.html#Compatibility"
    rel="Help"><strong>Compatibility:</strong></a> Apache 1.3.13
    and later, Win32 only 

    <p>Record HSE_APPEND_LOG_PARAMETER requests from ISAPI
    extentions to the query field (appended to the CustomLog %q
    component).</p>

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